Abstract
The 3H4 $ \to $ 3H5 transition of Thulium ions (Tm3+), which features laser emission at $\sim 2.3~\mu \text{m}$ is studied in details. We revise the conditions for efficient laser operation using a rate-equation model accounting for the ground-state bleaching, cross-relaxation and energy-transfer upconversion (ETU). We show that ETU has a crucial role in reaching more than unity pump quantum efficiency (QE) for $\sim 2.3~\mu \text{m}$ Tm lasers based on highly-doped crystals. A Ti:Sapphire pumped quasi-continuous-wave 3.5 at.% Tm:LiYF4 laser generated 0.73 W at 2306 nm with a record-high slope efficiency of 47.3% (versus the absorbed pump power, for double-pass pumping) featuring a QE of 1.27. Diode-pumping of this crystal yielded a peak output power of >2 W. The first $2.3~\mu \text{m}$ Tm waveguide laser is also reported based on Tm:LiYF4 epitaxial layers with even higher doping of 6.2 at.% generating 0.23 W with a slope efficiency of 19.8%. The spectroscopic properties of Tm:LiYF4 relevant for the $\sim 2.3~\mu \text{m}$ laser operation are revised as well.
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